1. Technical Field
This invention relates to transistor circuits for supplying constant current, and more particularly to a circuit using CMOS transistors of varying threshold voltages to provide a constant current source in an integrated circuit device.
2. Description of the Related Art:
In high speed integrated circuit devices such as microprocessors or application specific (ASIC) circuits, it is often desired to use a constant current supply in high speed logic circuits such as PLL, for example. It has been the practice to employ devices other than standard CMOS field-effect transistors to generate reference levels in these constant current sources. For example, p-n junction diodes or resistors, or combinations of these two, have been utilized for this purpose. The p-n junction diode would be formed by a P+ diffusion in an N-well, in one example. However, a CMOS process may not be compatible with use of a diode formed in this manner, because the shallow-diffusion p-n junction diodes may have a tendency to develop parasitic Schottky diodes in parallel with the p-n junction diode, resulting in creation of a large variation in the voltage drop across the diode structure and a high level of leakage. In addition to these problems with parasitic effects, the creation of analog components such as p-n junction diodes and resistors, i.e., components other than the standard CMOS P-channel and N-channel transistors, increases the process cost because additional steps must be added to the manufacturing method, and these additional steps not only add to the time and actions needed to complete the wafer processing, but also usually have a negative effect on yields. Accordingly, it would be preferable to provide a referen without using p-n junction diodes of this type, in a standard CMOS process.
Another factor in providing a current reference in a CMOS integrated circuit is the need to have the circuit exhibit a selected temperature coefficient. It is desirable that the device be relatively insensitive to temperature. The response of the current reference is a significant factor in overall system responsiveness to temperature. It is preferred that the current reference circuit be able to be adjusted in its temperature coefficient without major process revisions, i.e., that the adjustment be made without introducing steps in the process that are major variations from the standard process.